Light-Responsive Solid–Solid Phase Change Materials for Photon and Thermal Energy Storage

We report a series of adamantane-functionalized azobenzenes that store photon and thermal energy via reversible photoisomerization in the solid state for molecular solar thermal (MOST) energy storage. The adamantane unit serves as a 3D molecular separator that enables the spatial separation of azobenzene groups and results in their facile switching even in the crystalline phase. Upon isomerization, the phase transition from crystalline to amorphous solid occurs and contributes to additional energy storage. The exclusively solid-state MOST compounds with solid–solid phase transition overcome a major challenge of solid–liquid phase transition materials that require encapsulation for practical applications.


General Methods
All reagents and starting materials were purchased from commercial vendors and used as supplied unless otherwise indicated. All dry solvents were obtained from the solvent system and dry triethylamine was distilled from CaH2 and freshly used. All reactions were monitored by thin-layer chromatography (TLC) using Merck silica gel 60 F254 plates (0.25 mm). TLC plates were visualized using UV light (254 nm). Silica column chromatography was performed using Merck Silica Gel 60 (230-400 mesh). Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. and used as received. 1 H NMR and 13 C NMR were recorded on a Varian INOVA 400 spectrometer at 400 MHz. Chemical shifts are quoted in ppm relative to tetramethylsilane (TMS) using the residual solvent peak as the reference standard. ESI mass spectra were obtained on a Waters Quattro II ESI mass spectrometer.

UV-Vis Absorbance Spectroscopy
UV−Vis adsorption spectra of compounds 1-3 were obtained with a Cary 60 Bio UV−vis spectrophotometer in a UV Quartz cuvette with a path length of 10 mm. Compounds were dissolved in DMSO. The UV−vis absorption was first recorded in dark for 3-5 min, then samples were irradiated with a specified wavelength until no change in their absorbance was observed.

Thermal Half-life Measurements
Solutions of compounds 1-3 in DMSO were prepared and then irradiated at 365/340 nm to obtain a Z-rich state. The solutions were then heated at elevated temperatures in dark. The change in the concentration of the E isomer as a function of time was monitored, and the half-lives were obtained based on Eyring-Polanyi plots.
Differential Scanning Calorimetry (DSC) DSC analysis was conducted on a DSC 250 (TA Instruments) with an RSC 90 cooling component. All samples were heated to ~250 ℃ and cooled to −90 °C before reheating. E isomers of the compounds were heated and cooled at a rate of 4 phase transition.

Preparation of Z-isomer Samples for DSC Measurements
Z isomers were obtained by dissolving each E isomer in dichloromethane and irradiating the sample with an appropriate wavelength of light until the photostationary state was reached. Z-rich samples were concentrated, dried under high-vacuum, and then transferred to DSC pans for analysis. 1 H NMR spectra were taken before the DSC measurements to determine the percentage of Z isomers in the samples.

Thin Film Experiments
Thin-film samples were prepared by drop-casting 100 µL of 0.01 M DMSO solution of E isomers on a clean glass slide (2.5 x 2.5 cm 2 ) and heating them on a hot plate at 100 ℃ until the solvent completely evaporated. Then the sample was slowly cooled to room temperature. The temperature was controlled using a VWR Advanced hot plate stirrer. The thickness of the films was then measured using Zeta-20 Optical Profilometer.   UV-vis absorption spectra of (c) compound 1 and (d) compound 3 measured in thin films. Figure S2. Eyring-Polanyi plots of thermal Z-to-E isomerization of compounds 1-3 measured in DMSO. Table S1. Summary of thermal reversion activation energy (∆ ‡ , ∆S ‡ , ∆G ‡ ) and t1/2 of Z isomers of compounds 1-3 at 298 K.

Gas Adsorption Properties
Figure S14. N2 gas adsorption isotherms of E and Z isomers of compound 1-3 at 77 K.